An electrical transformer converts electricity at one voltage to electricity at another voltage, either of higher or lower value. This voltage conversion is achieved using coil assembly that includes a primary coil and a secondary coil in the transformer, each of which are wound on a ferromagnetic core and comprise a number of turns of an electrical conductor. The primary coil is connected to a source of voltage and the secondary coil is connected to a load. The coil assembly is mounted to a leg of the core, and the core includes first and second yokes connected by the leg. Certain transformers include multiple legs and a coil assembly on each leg.
For dry type transformers, such as those with coils for open windings, an air distance has to be maintained between the yokes and an adjacent end of the coil assembly to maintain dielectric clearance. In dry transformers, the high voltage conductor of the coil assembly is not embedded in resin. Thus, the conductor is exposed to air except for a thin turn insulation on the conductor surface. Sufficient air clearance prevents flashover from the high voltage conductor to the grounded yoke of the core. Thus, the leg of the core on which the coil assembly is mounted has to be of sufficient length to provide both a desired coil assembly length and to maintain this air distance between the yokes and the adjacent ends of the coil assembly.
Therefore, a design which allows the air distance between the ends of the coil assembly and the adjacent yoke to be lessened reduces overall cost and space requirements for the transformer since less core material is required and the length of the leg of the core on which the coil assembly is mounted can be reduced. However, such a design cannot negatively affect the performance of the transformer. It would therefore be desirable to provide a transformer with a reduced air clearance gap between the coil assembly and the yokes of the core. The present invention is directed to such a transformer.